iio: adc: stm32-adc: switch off running adc when going to low power

Switch off ADC when going to low power mode, in case it has been left
running in buffer mode. Then re-enable it when resuming.

Signed-off-by: Fabrice Gasnier <fabrice.gasnier@st.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>